1. Field of the Invention
The present invention relates generally to the use of radio frequency energy for heating and desiccating tissue. More particularly, the present invention relates to a control method, system, and apparatus for delivering radio frequency current to the tissue through electrodes disposed on bipolar surgical instruments.
The delivery of bipolar radio frequency energy to target regions within tissue is known for a variety of purposes. Of particular interest to the present invention, radio frequency energy may be delivered by bipolar surgical instruments to regions in target tissue for the purpose of heating and/or desiccation, referred to generally as hyperthermia. Bipolar electrosurgical devices rely on contacting electrodes of different polarity in close proximity to each other against or into tissue. For example, bipolar forceps have been used for coagulating, cutting, and desiccating tissue, where opposed jaws of the forceps are connected to different poles of an electrosurgical power supply. The radio frequency current thus flows from one jaw to the other through the tissue present therebetween. Use of such bipolar forceps is effective for a number of purposes and advantageous in that its effect is generally limited to the tissue held between the jaws without unwanted heating of adjacent tissues.
A primary goal for the delivery of bipolar radio frequency energy for hyperthermic treatments is the complete, thorough, and uniform heating of the treatment tissue without causing charring of the treatment tissue. Charring greatly increases electrical resistance through the tissue prematurely and can result in termination of the treatment before the tissue is uniformly heated. Uniform heating of the target tissue, however, can be difficult to achieve, particularly in highly vascularized tissues where the variability in local blood flow can have a significant effect on the heating characteristics of the tissue. For example, creation of a lesion in some highly perfused tissue locations may require twice as much power as an identically-sized lesion in less highly perfused locations. While a variety of approaches for achieving such complete, thorough, and uniform heating of tissue have been proposed, most such approaches are somewhat complex. In general, many approaches for achieving uniform tissue heating have relied on slow, low power level, gradual heating of the tissue to avoid the formation of charred or otherwise desiccated, high radio frequency impedance regions within the target tissue. Such approaches, however, are complex, can result in undesirable prolongation of the treatment, and are not always successful.
For these reasons, it would be desirable to provide improved treatment methods, systems, and apparatus which allow for effective and efficient delivery of radio frequency energy to target tissue using electrodes disposed on bipolar devices. In particular, it would be desirable to provide such methods, systems, and apparatus which are useful with many or all bipolar surgical instruments which are now available or which might become available in the future. The methods, systems, and apparatus should be simple to implement and use, and should preferably reduce the complexity, cost, and treatment time required to achieve complete, thorough, and uniform heating and/or desiccation of the target tissue without charring the target tissue. At least some of these objectives will be met by the invention described hereinafter.
The heating of tissue with radio frequency current using the preferred bipolar surgical systems of the present invention is described in co-pending application Ser. Nos. 09/071,689 filed May 1, 1998 and 09/303,007 filed Apr. 30, 1999, the full disclosures of which are incorporated herein by reference. Radio frequency power apparatus and methods are described in U.S. Pat. Nos. 5,954,717, 5,556,396; 5,514,129; 5,496,312; 5,437,664; and 5,370,645; WO 95/20360, WO 95/09577, and WO 93/08757. Bipolar electrosurgical devices are described in U.S. Pat. Nos. 5,833,690; 5,797,941; 5,702,390; 5,688,270; 5,655,085; 5,662,680; 5,582,611; 5,527,313; 5,445,638; 5,441,499; 5,403,312; 5,383,876; 5,217,460; 5,151,102; 5,098,431; 4,043,342; and 4,016,886; Soviet Union Patent Publication SU 197711; and French Patent No. 598,149.
The present invention provides improved methods, systems, and apparatus for effective and efficient delivery of radio frequency (RF) energy to electrodes of bipolar surgical instruments disposed in treatment tissue for inducing hyperthermia and other purposes. The treatment region resulting from bipolar radio frequency treatment may be located anywhere in the body where hyperthermic exposure may be beneficial. The treatment region may comprise and/or be located in tissue of or surrounding the liver, kidney, lung, bowel, stomach, pancreas, breast, uterus, prostate, muscle, membrane, appendix, other abdominal or thoracic organs, and the like.
Treatments according the present invention will usually be effected by passing a radio frequency current through the treatment tissue region in a bipolar manner where paired treatment electrodes are employed to both form a complete circuit and to uniformly and thoroughly heat tissue therebetween. The paired electrodes will have similar or identical surface areas in contact with tissue and geometries so that current flux is not concentrated preferentially at either electrode (or electrode component such as a tissue-penetrating needle) relative to the other electrode(s). Such bipolar current delivery is to be contrasted with xe2x80x9cmonopolarxe2x80x9d delivery where one electrode has a much smaller surface area and one or more xe2x80x9ccounterxe2x80x9d or xe2x80x9cdispersivexe2x80x9d electrodes are placed on the patient""s back or thighs to provide the necessary current return path. In the latter case, the smaller or active electrode will be the only one to effect tissue as a result of the current flux which is concentrated thereabout.
It has been found that the delivery of bipolar radio frequency power to electrodes disposed in tissue can, if the power is delivered for a sufficient time and/or at a sufficient power delivery level or flux, result in an increase in the electrical impedance between the electrodes and tissue. While such an increase in impedance is the natural consequence of tissue desiccation, it can be undesirable if it occurs prematurely since it results in an immediate fall-off of energy delivery (for a voltage limited radio frequency power source). Accordingly, the present invention relies on unique methods of radio frequency power delivery to uniformly and thoroughly heat the target tissue without charring the target tissue.
It is presently believed that the premature increase in electrode-tissue interface impedance may result from the formation of a thin gaseous or vapor layer over the electrode surfaces, apparently resulting from vaporization of water within the tissue as the temperature approaches the local boiling point. The thin gaseous layer appears to spread from an initial nucleation site to cover most or all of the electrode surfaces in a very short time period, resulting in the premature increase in electrode-tissue interface impedance which is very large when compared to the total system impedance prior to formation of the gaseous layer. The methods, systems, and apparatus of the present invention have been found to be useful and effective regardless of the actual mechanism which is responsible for the premature increase in impedance.
In a first particular aspect of the present invention, a method for heating a treatment region of tissue comprises introducing a bipolar surgical instrument, such as forceps, graspers, or the like, having first and second jaws with first and second electrode members within the treatment region. Tissue is grasped between the first and second jaws of the bipolar instrument. The electrode members are energized at a power level to deliver electrical energy to and heat tissue between the first and second electrode members. The power level is increased at a predetermined rate from an initial level. The initial level and predetermined rate are selected to avoid creating a vapor layer and to permit an impedance increase to occur as a result of complete tissue desiccation. The predetermined rate of power increase may be preselected by a user depending on the electrode sizes, the target tissue type, the degree of tissue perfusion, and the initial power level. Typically, the predetermined rate of power increase will be linear and increase at a rate in the range from 1 W/sec to 100 W/sec, preferably from 1 W/sec to 10 W/sec. Thus, the initial power level and predetermined rate of power increase allow for controlled delivery of bipolar radio frequency energy without premature impedance resulting from the formation of the thin gaseous layer. Furthermore, the increase in the power level at a predetermined rate from an initial level permits a natural impedance increase to occur as a result of complete tissue desiccation.
It may be further desirable to measure tissue impedance. The measured impedance may be compared to an impedance limit, wherein the impedance limit will be preselected by the user to indicate the impedance increase due to complete tissue desiccation, typically in the range from 50 ohms to 1000 ohms, preferably from 250 ohms to 750 ohms. If the measured impedance exceeds the impedance limit, the power delivery is automatically terminated. Additionally, the energizing of the electrode members and increasing the power level at a predetermined rate may be repeated at least once after termination until all the tissue between the electrode members is completely desiccated without charring the target tissue. Thus, complete heating and desiccation of tissue can be further optimized by monitoring tissue impedance and terminating the power delivery at a maximum impedance limit. As an added safety feature, the power delivery may also be automatically terminated after a preset amount of time, typically after period of 5 minutes, regardless of measured impedance so as to avoid overheating of the target tissue or unwanted heating of adjacent tissue. Alternatively, the power delivery may be manually terminated with the use of a radio frequency off switch that will immediately terminate power delivery.
In general, electrode members are energized with radio frequency energy supplied as a radio frequency current using a controlled voltage supply. The use of such radio frequency power sources is preferred because the limited voltage available decreases the likelihood of arcing or sparking from the electrode members into the tissue. Usually, the power supply will be operated at a level which depends on the electrode sizes, the target tissue type, and the degree of tissue perfusion. Typically, the power supply will provide power in the range from 5 W to 150 W, preferably from 10 W to 80 W, and a frequency in the range from 100 kHz to 2 MHz, preferably from 400 kHz to 500 kHz, during all phases of the above-described methods. The electrode members are energized typically for a time less than 5 minutes, preferably for a time in a range from 10 seconds to 1 minute. Additionally, at least one of the electrode members may comprise a plurality of tissue penetrating elements, the tissue penetrating elements engaging the tissue before the electrode members are energized.
In further aspects of the present invention, systems are provided which comprise an electrosurgical power supply, typically a radio frequency generator, in combination with written, electronic, or other instructions setting forth any of the methods set forth above.
In still another aspect of the present invention, computer programs embodied in a tangible medium, such as a floppy disk, compact disk, tape, flash memory, hard disk memory, read only memory (ROM), internet/modem instructions, and the like, may set forth any of the methods described above, in computer-readable code. Such computer programs are useful with digital controllers which may be built into a radio frequency generator or other electrosurgical power supply according to the present invention. Alternatively, such programs may be useful with general purpose computers, which can be interfaced with conventional electrosurgical power supplies for the control thereof according to any of the methods of the present invention.
In a still further aspect of the present invention, radio frequency generators are provided which comprise a radio frequency power source having both a controlled voltage output and a standard bipolar connection for bipolar forceps or the like. The radio frequency generator will further comprise means for automatically increasing power delivered to the bipolar connection. The increasing means increases the power at a predetermined rate from an initial level. The initial level and predetermined rate avoid formation of a vapor layer while permitting an impedance increase to occur as a result of complete tissue desiccation. The predetermined rate of power increase will be preselected by a user depending on the electrode sizes as well as the initial power level, typically being linear and increasing at a rate in the range from 1 W/sec to 100 W/sec, preferably from 1 W/sec to 10 W/sec.
The increasing means may initiate a cycle where it measures an impedance of tissue, compares the measured impedance to an impedance limit, and increases the power level based on the predetermined rate of power increase if the measured impedance does not exceed the impedance limit. The impedance limit is selected to indicate the impedance increase due to complete tissue desiccation, typically in the range from 50 ohms to 1000 ohms, preferably from 250 ohms to 750 ohms. Furthermore, the increasing means repeats the cycle and need only be activated once for continual cycling. The increasing means may comprise a programmable digital controller, a control program embodied in a tangible medium, or other means for automatically increasing power delivered by the generator. In particular, the digital controller or other increasing means can be programmed to implement any of the methods described above independent of operator intervention.
The radio frequency generator may further comprise a user interface for inputting the rate of power increase, the initial power level, and an impedance limit. The user interface may further comprise a front panel display that displays at least one of the following: a real-time impedance, total energy delivered, and/or instantaneous power delivered. Optionally, the user interface may comprise an audible alarm which indicates the current delivery of power in addition to appropriate generator status signals.